If this is what I think it is (please post a photo if you want us to verify) then this is port-like wine from South Africa, and is a 30yr tawny-style wine. If so then it will probably have been bottled in 1979, and while intended for immediate drinking at that time, should still be drinkable now as long as it has been well stored.

In peak condition this might sell for around £60-80 by private sale or by auction after fees removed; depending on condition it might fetch much less; you might be able to sell it for more next year when it will be an anniversary year. I would suggest saving it until next year and then drinking it with a friend or relative whose birth date ends in a 9.

It should be drinkable. I had a bottle of the 1963 version a few years ago and it was still very much alive — very acidic, with the acidity being the feature which kept the port alive. I would expect this vintage to be pretty impressive.